Need help with a 1968 Fender Vibrochamp

[taherbert]

So I went a little gear crazy this weekend, and I bought a 1968 Fender Vibrochamp and a 1960's amp labeled "Supreme" (whole other story, basically a Silvertone). Anywho, the Vibrochamp sounded nice in the store, but when I got it home and started putting it through its paces, I noticed that anytime I play two low strings at once, e.g. a G chord, I get this ugly farty phase canceling sound. The highs and mids sound great, but this thing isn't playable as it is. This mp is in near perfect condition, and has had the stock speaker exchanged for a Weber Signature Series.

 

So my question is should I take it back to the store, is this what I should expect from a champ like this, or is this a simple (i.e. cheap fix)? I haven't swapped out the tubes because I don't have replacements on hand, and I don't want to buy them for an amp I may not keep. If this is definitely a tube thing, I'd be up for it though. Any help is appreciated.

[taherbert]

bump

[RupertB]

Tell us some more about the "ugly farty phase-cancelling noise."

You're never going to get great bass response out of a Champ/VC, but if what you're describing is a nasty sounding hard-clip distortion, there are some things you can do (or can have done).

My VC had just that kind of overdrive when I got it. I did a cap job on it and took some readings. The voltages were 18-20v higher than what I saw on the schematics. Also, when I calculated plate dissipation, I was getting ~50ma. Way too high. I increased the stock cathode resistor from 470 to 720 and had already replaced the cathode cap with one rated for 50v (from 25v) which I recommend if you do this mod.

The result is a plate dissipation in the high 20s, a little more clean headroom, and an overdrive sound that is much sweeter, smoother, and less hard-clip distortion than before.

Its an easy mod if you're handy with a soldering iron & easily reversible, or one you can get a tech to do for an hour's bench fee.

Also, check your 5Y3 rectifier. If yours is a new production version (which may pass more current than NOS), you may find that an NOS rectifier tames things a bit a well.

[guitarcapo]

So I went a little gear crazy this weekend, and I bought a 1968 Fender Vibrochamp and a 1960's amp labeled "Supreme" (whole other story, basically a Silvertone). Anywho, the Vibrochamp sounded nice in the store, but when I got it home and started putting it through its paces, I noticed that anytime I play two low strings at once, e.g. a G chord, I get this ugly farty phase canceling sound. The highs and mids sound great, but this thing isn't playable as it is. This mp is in near perfect condition, and has had the stock speaker exchanged for a Weber Signature Series. So my question is should I take it back to the store, is this what I should expect from a champ like this, or is this a simple (i.e. cheap fix)? I haven't swapped out the tubes because I don't have replacements on hand, and I don't want to buy them for an amp I may not keep. If this is definitely a tube thing, I'd be up for it though. Any help is appreciated.

 

 

I had an old Vibro Champ with the same problem. I did a lot of things and could never get reid of the problem. I swapped out the 8" speaker for a 10" Weber. I swapped out some caps in the tone stack. I replaced tubes. I even replaced the tube rectifier with a solid state plug-in and tried replacing the

6V6 power tube with a 6L6. In retrospect I think the problem might have been somewhere in the vibrato circuit, and I never though about modding it to disconnect that part. The reason I say that is that I have a tweed Champ clone that I was easily able to mod for awesome tone and that's one major difference between the two.

 

Vibro Champs get a lot of love on these message boards but I ended up selling mine. I came to the conclusion that I would have had to mod it too much away from stock for it to be worth the effort.

 

Some other areas you might want to look at are the caps and resistors at the cathode of each tube, of course the filter caps, and maybe making sure that the caps in the tone stack don't need to be replaced because of drifting way off spec.

[silverfacechamp]

So I went a little gear crazy this weekend, and I bought a 1968 Fender Vibrochamp and a 1960's amp labeled "Supreme" (whole other story, basically a Silvertone). Anywho, the Vibrochamp sounded nice in the store, but when I got it home and started putting it through its paces, I noticed that anytime I play two low strings at once, e.g. a G chord, I get this ugly farty phase canceling sound. The highs and mids sound great, but this thing isn't playable as it is. This mp is in near perfect condition, and has had the stock speaker exchanged for a Weber Signature Series. So my question is should I take it back to the store, is this what I should expect from a champ like this, or is this a simple (i.e. cheap fix)? I haven't swapped out the tubes because I don't have replacements on hand, and I don't want to buy them for an amp I may not keep. If this is definitely a tube thing, I'd be up for it though. Any help is appreciated.

 

 

That amp has one of the simplest circuits out there, so it'll be comparatively cheap to fix. Take it to a tech and and have him check it out. You probably have a something out of spec somewhere. I wouldn't get rid of it....it's way too cool of an amp and way to easy to get it fixed.

[MalcolmTex]

I was gonna suggest a Weber, but it's already been swapped out. I agree with the posts. Once it's up and running, you're gonna love your VC. I would never get rid of mine. It's great to record with and I run it into a 2 12" for gigging. Stick a tube screamer in front and it's tone for days. Keep it, get it serviced and you'll be glad you did.

[taherbert]

Thanks for the responses, guys. I think I'm going to keep it, but as far as more info on the problem:

When the amp is cranked to the point where the lower strings are breaking up in that great Fender way, playing any interval on the top 2 strings less than a 5th basically cancels out one of the notes and makes a really unpleasant kind of LFO-type square wave. Basically distorting until it becomes somewhat atonal.

It's better if I reduce the gain to like a 7 or 8 or turn the bass all the way down, but then I either can't get the overdrive I want on the rest of the strings or the tone sucks. Turning the vibrato depth and and rate all the way down doesn't help. I don't have a pedal to switch it off. It's a lot more noticeable on the high gain input than the low gain input, but still there on the low. I dunno if that helps pinpoint the problem any better.

[silverfacechamp]

Thanks for the responses, guys. I think I'm going to keep it, but as far as more info on the problem: When the amp is cranked to the point where the lower strings are breaking up in that great Fender way, playing any interval on the top 2 strings less than a 5th basically cancels out one of the notes and makes a really unpleasant kind of LFO-type square wave. Basically distorting until it becomes somewhat atonal. It's better if I reduce the gain to like a 7 or 8 or turn the bass all the way down, but then I either can't get the overdrive I want on the rest of the strings or the tone sucks. Turning the vibrato depth and and rate all the way down doesn't help. I don't have a pedal to switch it off. It's a lot more noticeable on the high gain input than the low gain input, but still there on the low. I dunno if that helps pinpoint the problem any better.

 

 

Take it to a tech. It's nothing you can fix yourself if you're not familiar with amp repair.